Media Sheet Guide Device and Method for Maintaining Proper Sheet Alignment in an Image Forming Machine

ABSTRACT

A media sheet guide device for maintaining proper sheet alignment in an image forming machine includes a body extending between spaced apart reference edges of respective upstream and downstream media alignment mechanisms and a face on the body defining a path along which a sheet moves lengthwise from the upstream reference edge to the downstream reference edge. The face has a section extending widthwise and defining a portion of the path about which the sheet bends widthwise as the sheet moves lengthwise along the path portion. The face section has first and second segments differing in curvature for applying a differential drag widthwise across the sheet causing a rotation of the sheet sufficiently to displace a leading end of the sheet away from the downstream reference edge to thereby ensure that a leading corner of the sheet can enter into the downstream reference edge without jamming against an entry end thereof.

CROSS REFERENCES TO RELATED APPLICATIONS

None.

BACKGROUND

1. Field of the Invention

The present invention relates generally to electrophotographic (EP) image forming machines and, more particularly, to a media sheet guide device and method for maintaining proper sheet alignment in the EP image forming machine.

2. Description of the Related Art

In an EP image forming machine, such as a laser printer, having a duplexer, each sheet of media has to be repositioned relative to a desired location in the media return path of the duplexer prior to the sheet being returned into the media input path of the printer. Repositioning of the sheet is required due to the possibly of it becoming misaligned during its initial advancement through the printer when printing occurs on its first side. Misalignment may be to the left or right of the desired location of the sheet. The sheet also may be angled or skewed relative to its desired location. To have accurate positioning of each sheet during its return to the printing area of the printer so that printing on its second side is accurate, the sheet must be accurately repositioned relative to a known location or surface, such as defined by a reference edge extending generally parallel to the paper path. See U.S. Pat. No. 6,840,514 assigned to the assignee of the present invention for a discussion of these issues and for disclosure of a media alignment mechanism as one approach for their resolution.

In various prior art arrangements to align a sheet of media with a reference edge to its correct position and orientation, alignment rollers have been employed. These alignment rollers are skewed so that they apply both a force perpendicular to the reference edge and a force parallel to the reference edge to advance the sheet in a desired alignment. In printers that utilize reference edge alignment, successively positioned upstream and downstream media alignment mechanisms made up of skew rollers and reference edges can be found at various areas in the printers, one such area being between the image formation path of the base printer and the duplex path. It is desired that the reference edges in these successively positioned media alignment mechanisms be placed together as close as possible (i.e. lie in the same theoretical plane) to eliminate sheet skew as the sheet passes from one mechanism to the next.

One problem with this arrangement is that if the reference edges of the successive media alignment mechanisms are located far apart, such as at a distance D>50 mm, in the downstream sheet path, when the leading edge of the sheet enters the entrance to the next reference edge, it could engage the entry end of the reference edge and create a paper jam if there is not enough offset distance between the two reference edges involved in the hand-off. This is because a very large offset may be required to account for physical tolerances between the successive sheet alignment mechanisms and any walking or steering imparted into the sheet by the feed system. Also, in the case of the duplex path, the position and orientation of the sheet is not known when the leading edge of the sheet enters the entrance to the reference edge in its duplex operation. Thus, it is necessary to ensure that the sheet does not engage the entry end of the reference edge when the sheet is to be printed on its second side.

Thus, there is a need for an innovation that will address and satisfactorily solve the aforementioned problems.

SUMMARY OF THE INVENTION

The present invention meets this need by providing an innovation for maintaining proper alignment of media sheets in an image forming machine through introducing or applying differential drag across each of the media sheets to bring about its proper alignment between successive media alignment mechanisms. In such manner, the present invention solves the design paradox by providing a way of minimizing offset between reference edges of upstream and downstream media alignment mechanisms of an image forming machine for good skew performance, while allowing sheets to pass between the mechanisms without jamming into the reference edge downstream.

Accordingly, in an aspect of the present invention, a media sheet guide device for maintaining proper sheet alignment in an image forming machine includes a body extending between spaced apart reference edges of respective upstream and downstream media alignment mechanisms in the machine, and a face on the body defining a path along which a sheet moves lengthwise from the reference edge of the upstream media alignment mechanism to the reference edge of the downstream media alignment mechanism. The face has a widthwise section defining a portion of the path and about which the sheet bends widthwise as the sheet moves lengthwise along the portion of the path. The section of the face has first and second segments differing in curvature for applying a differential drag widthwise across the sheet as the sheet bends widthwise about the section of the face and moves lengthwise along the portion of the path. The application of the differential drag across the sheet causes a rotation of the sheet sufficiently to displace a leading end of the sheet away from the reference edge of the downstream media alignment mechanism, thereby ensuring that a leading corner of the sheet can enter into the reference edge of the downstream media alignment mechanism without jamming against an entry end of the downstream reference edge.

In another aspect of the present invention, a method for maintaining proper sheet alignment in an image forming machine includes defining a path along which a sheet moves lengthwise from an upstream reference edge of an upstream media alignment mechanism to a downstream reference edge of a downstream media alignment mechanism and applying a differential drag widthwise across the sheet causing a rotation of the sheet sufficiently to displace a leading end of the sheet away from the downstream reference edge to thereby ensure that a leading corner of the sheet can enter into the downstream reference edge without jamming against an entry end thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. 1 is a schematic representation of a prior art printer with respect to which modifications in accordance with the present invention can be applied.

FIG. 2 is a plan layout diagram of respective media alignment mechanisms located at opposite ends of a return path of FIG. 1 illustrating a problem caused by the provision of a minimum offset between spaced reference edges of the respective media alignment mechanisms.

FIG. 3 is a plan layout diagram similar to that of FIG. 2 but of the solution brought about by the present invention.

FIG. 4 is a perspective view of the media sheet guide device having features for implementing the solution of the present invention depicted in the diagram of FIG. 3 for applying differential drag across the media sheet to maintain its proper alignment.

FIG. 5 is an enlarged schematic representation of a return path extending between and interconnecting an image formation path and a duplex path of the printer but now incorporating the media sheet guide device of FIG. 4 in the return path.

FIG. 6 is a diagram of the features of the present invention implemented by the media sheet guide device of FIG. 4.

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numerals refer to like elements throughout the views. Furthermore, terms such as “lengthwise” and “widthwise” are used for the purpose of facilitating an understanding of the various aspects of the invention and do not to imply relative sizes of such aspects; terms such as “fore-and-aft” and side-to-side? could just as readily be used in place of “lengthwise” and “widthwise”.

Referring now to FIG. 1, there is schematically represented a prior art image forming machine in the form of a laser printer, generally designated 10. The printer 10 includes a housing 12 and a media input tray 14 removably disposed in the housing 12 through a lower front opening 16 in the housing 12 and providing a source of media sheets for the operations of the printer 10. The input tray 14 is sized to hold a first stack 18 of the sheets. A sheet pick mechanism 20 also disposed in the housing 12 picks and feeds the sheets from the tray 14 and along a ramp 22 that directs the sheets further along toward an entrance end 24 a of an image formation path 24. The pick mechanism 20 may includes a roller 26 positioned on a pivoting arm 28. The pivoting arm 28 causes the roller 26 to remain in contact with a topmost sheet on the first stack 18. Rotation of the roller 26 removes the sheet from the stack 18 with the leading edge contacting the ramp 22 so that the sheet will thus be moved toward the entrance end 24 a of the image formation path 24.

A multi-purpose feeder 30 may also be included in the housing 12 of the printer 10 to move additional sheets toward the entrance end 24 a of the image formation path 24. The multi-purpose feeder 30 includes a support floor 32 adjacent to another ramp 34. An additional sheet pick mechanism 36 is provided, having a pivoting arm 38 and a roller 40 thereon. Rotation of the roller 40 moves the sheet from the support floor 32 up the ramp 34 toward the entrance end 24 a of the image formation path 24. The additional sheet pick mechanism 36 within the multi-purpose feeder 30 may be the same or different from the sheet pick mechanism 20 associated with the media input tray 14.

The operation of the printer 10 is conventionally well-known. After a media sheet is introduced through the input tray 14 or the multi-purpose feeder 30, the sheet is presented at the entrance end 24 a of the image formation path 24 as defined by a nip 42 of a media alignment mechanism 44 formed between its set of drive roller 46. The media alignment mechanism 44 removes lateral skew from the sheet and precisely times its passage on to the image forming stations 48 located downstream along the image formation path 24.

After the sheet passes the media alignment mechanism 44 it contacts a transport belt 50, which carries the sheet along the image formation path 24 past successive photoconductor units 52 of the image forming stations 48. At each successive photoconductor unit 52, a latent image is formed by an imaging device 54 and optically projected onto a photoconductive member. The latent image is developed by applying toner to the photoconductive member from a toner reservoir. The toner is subsequently deposited on the sheet as it is conveyed past each of the photoconductor units 52 by the transport belt 50.

The toner is then thermally fused to the sheet by a fuser 56 and the sheet then passes through an exit end 24 b of the image formation path 24 to a media sheet directing mechanism in the form of a pair of reversible exit rollers 58 which feed the media sheet face down onto a media sheets collection site 60 on a top exterior portion 62 of the printer housing 12. Alternatively, the exit rollers 58 may reverse motion after the trailing edge of the sheet has passes an inlet end 64 a to a duplex path 64. The reversible exit rollers 58 then direct the sheet through the inlet end 64 a of the vertically-oriented duplex path 64 to where the sheet falls by gravity through an internal structure 66 defining the duplex path 64 to an outlet end 64 b of the duplex path 64. From there, the sheet then travels along a return path 68 defined between a pair of upper and lower sheet turn-around guides 70, 72 mounted on the tray 14. The guides 70, 72 interconnect the outlet end 64 b of the duplex path 64 with the nip 42 at the entrance end 24 a of the image formation path 24. The sheet is transported by the transport belt 50 through the image formation path 24 where it is processed for the printing of another image on the back side of the sheet. The double-sided media sheet is then delivered face down to the collection site 60 on the printer housing 12.

Turning now to FIG. 2, a simplified diagram is shown of the downstream media alignment mechanism 44 at the entrance end 24 a to the image formation path 24, as seen in FIG. 1, and an upstream media alignment mechanism 74 at the exit end 64 b of the duplex path 64, as also seen in FIG. 1, so as to define the return path 68 for movement of a sheet 80 therebetween. The upstream media alignment mechanism 74 includes an upstream reference edge 76 and a set of skew rollers 78 to place the sheet 80 in a known position prior to entry into the return path 68 for subsequent moving toward entrance end 24 a of the image formation path 24 defined by the downstream media alignment mechanism 44. The downstream media alignment mechanism 44 in addition to its set of drive rollers 46 includes a downstream reference edge 82 that performs final media sheet alignment prior to imaging that occurs along the image formation path 24. There is a very long sheet path length or distance, enclosed by the upper and lower guides 70, 72, between the upstream reference edge 76 and the downstream reference edge 82. In this particular arrangement, the length or distance D of the media return path 68 is greater than 100 mm. The upstream media alignment mechanism 74 also may include an intermediate feed device 84 after the upstream reference edge 76 and set of rollers 78. However, due to cost and space considerations, there is usually no reference edge or mechanism between the upstream media alignment mechanism 74 and the downstream media alignment mechanism 44 to support, drive or guide the edge of the sheet 80.

FIG. 2 also illustrates the primary problem with this arrangement. It is desired to have a minimum offset O between the upstream reference edge 76 and the downstream reference edge 82. This is to minimize the skew imparted into the sheet 80 due to the torque developed on the sheet 80 from having to align to the downstream reference edge 82 through the offset distance O. However, if the offset distance O is small and the distance D between the upstream and downstream reference edges 76, 82 is large, effects of sheet path drag, sheet walking, sheet steering, and tolerances can cause the sheet 80 to rotate as shown in FIG. 3 and crash and jam at the entry end 82 a of the downstream reference edge 82.

As can be understood in FIG. 3, to solve this problem and allow for a minimum reference edge offset, predictable differential drag can be introduced into, or applied to, the sheet 80 by the design of the sheet return path 68. Consider that the sheet 80 has a centerline 80 a, and a non-reference edge side 80 b and a reference edge side 80 c on opposite sides of the centerline 80 a and relative to the locations of the upstream and downstream reference edges 76, 82. Prior to the sheet 80 entering the downstream reference edge 82, a differential drag is introduced on the non-reference edge side 80 b of the sheet 80 relative to the centerline 80 a. This differential drag causes the leading end 80 d of the sheet 80 to rotate away from the downstream reference edge 82 and allows a leading corner 80 e of the sheet 80 to enter into the downstream reference edge 82 without crashing or jamming against the entry end 82 a thereof. Then, as the sheet 80 passes through the set of downstream drive rollers 46, the previous rotation of the sheet 80 is removed as the sheet is brought into alignment with the downstream reference edge 82. For this particular application, ˜200 grams of differential drag is applied to the sheet to produce 1-2 mm of rotation in the leading end 80 d of the sheet 80, although other differential drag amounts may be found to work depending on the particular media set supported or utilized by the printer 10.

The addition of differential drag can be customized based on the media set supported by the printer 10. For the example shown in FIG. 3, the supported media set of the duplex mode ranges in width from A5 to Letter. To ensure that the media set obtains the needed rotation in the leading end 80 d of the sheet 80, differential drag is introduced in a position for A5 media and for letter media. This covers those two media and all of those in between from a width perspective. It also should be noted that the differential drag can be added at any position between the upstream and downstream reference edges 76, 82 in the direction of the sheet feed. However, it is found that adding the differential drag just prior to the downstream nip 42 is most effective.

Referring now to FIGS. 4-6, there is illustrated a media sheet guide device, generally designated 86, which includes features in accordance with the present invention by which the differential drag is physically introduced or applied to the sheet 80 moving through the return path 68. The guide device 86 includes a body 88 that replaces the upper guide 70 and has a face 90 defining the return path 68 and about which media sheets 80 bend widthwise as they move lengthwise from the upstream media alignment mechanism 74 to the downstream media alignment mechanism 44. There are no reference edges nor alignment mechanisms between the upstream and downstream mechanisms 74, 44 to support, drive or guide the edge of each of the sheets 80. The face 90 of the body 88 has a widthwise section 90 a located at a downstream end portion 88 a of the body 88 that is located closest to the downstream nip 42 of the downstream media alignment mechanism 44 and defines a portion 68 a of the return path 68. The widthwise section 90 a of the face 90 has first and second segments 92, 94, positioned side-by-side and spaced apart, that differ in curvature lengthwise for applying the desired differential drag widthwise across the sheet 80 as the sheet bends widthwise about the widthwise section 90 a of the face 90 and moved lengthwise along the portion 68 a of the return path 68. The application of the differential drag causes a rotation of the sheet 80 sufficiently to displace the leading end 80 d of the sheet 80 away from the downstream reference edge 76, thereby ensuring that the leading corner 80 e can enter into the downstream reference edge 82 without crashing and jamming against the entry end 82 a of the downstream reference edge 82.

More particularly, the media sheet guide device body 88 includes a base 96 and a plurality of guide elements in the form of guide ribs 98 formed on an underside 96 a of the base 96 and arranged to extend lengthwise of the base 96 and generally parallel to one another in the direction of sheet movement. The guide ribs 98 are spaced apart laterally from one another across the width or widthwise of the base 96 and have respective thicknesses less than the distance between adjacent ones of the guide ribs 98. The outer edge surfaces 98 a of the guide ribs 98 collectively define the face 90 of the body 88. Particularly, downstream end portions 98 b of the guide ribs 98 on the downstream end portion 88 a of the body 88 having curvatures that cause the sheet 80 to turn nearly 90° before reaching the downstream nip 42 and downstream reference edge 76 of the downstream media alignment mechanism 44. Thus, each sheet 80 will bend about or around the lengthwise curved guide rib end portions 98 b as the leading end 80 d of the sheet 80 approaches and moves into the nip 42. The guide ribs 98 may be spaced apart laterally from one another by substantially the same amount of distance and have the same heights to which they extend outwardly from the base 90.

However, for applying the desired differential drag on the sheet 80 as the sheet 80 passes and bends around the downstream end portion 88 a of the body 88, between selected ones of the guide ribs 98 at the downstream end portions 98 a thereof one or more fillers are installed to provide, in effect, rib extensions 100 which modify or change the lengthwise curvatures of the downstream end portions 98 a of the selected guide ribs 98. The locations of and additional increments of height added by these rib extensions 100 are calculated to introduce or apply the desired differential drag to the media sheet 80 such as will bring about rotation of the leading end 80 d of the sheet 80 sufficiently away from the downstream reference edge 76 of the downstream media alignment mechanism 44 so as to avoid jamming of the leading corner 80 e of the sheet with the entry end 76 a of the downstream reference edge 76 as the sheet 80 approaches the downstream nip 42 of the mechanism 44 and the entrance end 24 a of the image formation path 24. Thus, the desired differential drag is physically introduced into the downstream portion 68 a of the sheet return path 68 by altering or modifying the rib geometry at a location just prior to the downstream reference edge 76 of the downstream media alignment mechanism 44. In the portion of the duplex re-entry or return path 68 at the location of the downstream end 88 a of the guide device body 88 defined by the curvature of the guide ribs 98, the sheet 80 is nominally fed through a 45 mm turn radius defined thereon. The sheet 80 is guided and supported through this area by the downstream end portions 98 b of the ribs 98 (located within the first segment 92 of the face 90) that allow the sheet to take or bent through this radius (see the long dash/short dash line in FIG. 6). In the A5 and letter width positions, the downstream end portions 98 b of the selected ones of the sheet support or guide ribs 98 are modified by the presence of the rib extensions 100 (located within the second segment 94 of the face 90) to make the media sheets 80 to take, or bend around, a different, tighter arcuate lengthwise shape or curvature through this area (see the dot/dash line in FIG. 6). In this particular embodiment, the bend radius of the rib extensions 100 of the selected ribs 98 through the non-reference edge side 80 b of the sheet 80 is 35 mm. Since the non-reference edge side 80 b of the sheet 80 has to go through a tighter bend radius, there is more drag on that side 80 b of the sheet 80 versus the drag on the reference edge side 80 c of the sheet 80. This differential drag is achieved by forcing the sheet 80 through a tighter bend radius on one edge side of the sheet than the other. The tighter bend radius always is provided on the opposite side of the centerline 80 a of the sheet from the reference edge. This differential drag develops a force sufficient to impart a slight amount of rotation of the sheet 80 in counterclockwise direction, as viewed in FIG. 3, which is sufficient to allow or ensure the sheet 80 to cleanly enter into the space adjacent to inner face 76 a of the downstream reference edge 76. The leading corner 80 e of the sheet 80 does not crash into nor jam upon the downstream reference edge 76. It should be noted that other bend radii combinations may be found that can impart this differential drag to the sheet return path 68.

To recap, the above-described method of reliably handing off media sheets applies to any two reference edges in the printer where the offset between the edges is minimal. The rib extensions 100 on the ribs 98 at the downstream end portions 98 b of selected one of the ribs 98 provide the differential bend radii across the width or widthwise of the sheet 80. Also, the rib geometry provides differential drag widthwise across the sheet and acting in a direction parallel to sheet feed direction of movement, while minimizing additional drag lengthwise of the sheet and acting in a direction perpendicular to the feed direction, so that downstream media alignment can be performed more reliably.

The foregoing description of several embodiments of the invention has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto. 

1. A media sheet guide device for maintaining sheet alignment in an image forming machine, comprising: an upper guide having a curved face; and a lower guide having a curved opening therein, said upper guide positioned within the curved opening, said upper and lower guides forming a curved path therebetween along which a sheet moves; said upper guide comprising: a body extending between spaced apart reference edges of respective upstream and downstream media alignment mechanisms in a image forming machine; and a face on said body defining said curved path along which a sheet moves lengthwise from the reference edge of the upstream media alignment mechanism to the reference edge of the downstream media alignment mechanism; said face having a section extending widthwise and defining a portion of said curved path about which the sheet bends widthwise as the sheet moves lengthwise along said portion of said curved path, said section of said face having first and second segments differing in curvature for applying a differential drag widthwise across the sheet as the sheet bends widthwise and moves lengthwise along said portion of said curved path, said application of said differential drag across the sheet causing a rotation of the sheet sufficiently to displace a leading end of the sheet away from the reference edge of the downstream media alignment mechanism, thereby ensuring that a leading corner of the sheet can enter into the reference edge of the downstream media alignment mechanism without jamming against an entry end of the reference edge of the downstream media alignment mechanism.
 2. The device of claim 1 wherein said first segment of said section of said face aligns with the sheet along a first portion thereof at a first side of a centerline of the sheet, said second segment of said section of said face aligns with the sheet along a second portion thereof at a second side of the centerline of the sheet opposite from the first side of the sheet such that the first segment is displaced farther than said second segment from the reference edge of the downstream media alignment mechanism.
 3. The device of claim 2 wherein said curvature of said first segment of said section of said face has a first bend radius and said curvature of said second segment of said section of said face has a second bend radius longer than said first bend radius such that greater drag is applied on the sheet at the first portion thereof by said first segment of said face located more remote from the downstream reference edge than at the second portion thereof by said second segment of said face located more adjacent to the reference edge of the downstream media alignment mechanism.
 4. The device of claim 1 wherein said curvature of said first segment of said face has a first bend radius and said curvature of said second segment has a second bend radius longer than said first radius such that greater drag is applied on the sheet farther from the reference edge of the downstream media alignment mechanism by said first segment of said face than by said second segment of said face.
 5. The device of claim 4 wherein said first segment of said face is disposed at different locations widthwise along said section of said face depending upon which one of different sizes of sheets the differential drag is to be applied.
 6. A media sheet guide device for maintaining sheet alignment in an image forming machine, comprising: an upper guide having a curved face; and a lower guide having a curved opening therein, said upper guide positioned within the curved opening, said upper and lower guides forming a curved path therebetween along which a sheet moves; said upper guide comprising: a body extending between spaced apart referenced edges of upstream and downstream media alignment mechanisms in said machine and having a face defining said curved path along which a sheet moves from the upstream reference edge to the downstream reference edge; a base of said body extending between the upstream and downstream reference edges; and a plurality of guide ribs formed on said base so as to define at least a portion of said face of said body, said guide ribs being laterally spaced apart widthwise of said base and having heights through which they protrude outwardly from said base, said guide ribs also having respective downstream end portions with curvatures extending lengthwise in the direction of the path and about which downstream end portions the sheet bends as the sheet approaches the downstream reference edge, selected ones of said downstream end portions of said guide ribs having at least one extension which protrudes outwardly from said guide rib end portions so as to modify said lengthwise extending curvatures thereof such that a differential drag is applied to the sheet that causes rotation of the sheet sufficient to displace a leading end thereof away from the downstream reference edge ensuring that a leading corner of the sheet may enter into the downstream reference edge without jamming against the downstream reference edge.
 7. The device of claim 6 wherein said selected ones of said guide rib end portions having said extension aligns with the sheet along a first portion thereof at a first side of a centerline of the sheet and at least one of said guide rib end portions without said extension aligns with the sheet along a second portion thereof at a second side of the centerline of the sheet opposite from the first side of the sheet such that said selected guide rib end portions with said extension is displaced farther from the downstream reference edge than said at least one of said guide rib end portions without said extension.
 8. The device of claim 7 wherein said curvature of said downstream end portions of said selected ones of said guide ribs with said extension has a first bend radius and said curvature of said guide rib end portions without said extension has a second bend radius longer than said first bend radius such that greater drag is applied on the sheet at the first portion thereof located more remote from the downstream reference edge than at the second portion thereof located more adjacent to the downstream reference edge.
 9. The device of claim 6 wherein said curvature of said downstream end portions of said selected ones of said guide ribs with said extension has a first bend radius and said curvature of said guide rib end portions without said extension has a second bend radius longer than said first radius such that greater drag is applied on the sheet farther from the downstream reference edge by said guide rib end portions with said extension than by said guide rib end portions without said extension.
 10. The device of claim 9 wherein said selected ones of said guide rib end portions with said extension are disposed at different locations widthwise of said base depending upon to which one of different sizes of sheets the differential drag is to be applied.
 11. The device of claim 6 wherein said guide ribs are spaced apart from one another by substantially the same amount of distance.
 12. The device of claim 11 wherein said heights at which said guide ribs protrude outwardly from said base are substantially the same.
 13. A method for maintaining proper sheet alignment in an image forming machine, comprising: defining a path along which a sheet moves lengthwise from an upstream reference edge of an upstream media alignment mechanism to a downstream reference edge of a downstream media alignment mechanism; and applying a differential drag widthwise across the sheet causing a rotation of the sheet sufficiently to displace a leading end of the sheet away from the downstream reference edge to thereby ensure that a leading corner of the sheet can enter into the downstream reference edge without jamming against an entry end thereof.
 14. The method of claim 13 wherein said applying the differential drag includes applying a greater drag to the sheet along a first portion of the sheet at a first side of a centerline of the sheet and a lesser drag to the sheet along a second portion of the sheet at a second side of the centerline of the sheet opposite from the first side of the sheet.
 15. The method of claim 14 wherein the first portion of the sheet where the greater drag is applied is located farther from the downstream reference edge than the second portion of the sheet where the lesser drag is applied. 